This fails in idStackWindows at the OS call to HostByAddress - for some
reason, a machine name is not returned, and a Socket error 11004 is thrown.
(This terminates the thread.)
I'm still wondering why, but for the time beeing I've done a quick and dirty
fix by wrapping that statement, and another HostByAddress statement further
down, in a try..except block

Ideas, anyone? And what are the side effects of this "fix"? As far as I can
see, only the informational value of the resulting header ("Mail from:xxx"
and so on) is lost?